Thermally coupled image amplifier using internal feedback



Nov. 18, 1969 B. B. SNN/m 3,479,515

THERMALLY COUPLED IMAGE AMPLIFIER USING INTERNAL FEEDBACK F'iled March25, 1965 IN VEN TOR. 5in/.www 5ml/ar United States Patent O U.S. Cl.Z50-213 9 Claims ABSTRACT OF THE DISCLOSURE A method and device in whicha photoconductor is used to convert an electromagnetic-radiation image(e.g., in the visible, or near-visible spectrum) to patterns of electriccurrent, the heat generated by the current pattern thereafter being usedto quench or stimulate the fluoresence of an externally-excited phosphorto reform the original image in fluorescent light. Increased contrast isobtained in the iiuorescent image by means of an internally positionedelectroluminescent material which respods to the electric current tofeed-back additional luminescence to the photoconductor, providing afurther increase in current and, thereby providing an intensification ofthe image-quenching (or -intensifying) heat patterns.

This invention relates to improvements in image arnpliers and moreparticularly to image amplifiers employing thermosensitive phosphors thestimulated light emission of which is either quenched or intensied bythe application of heat.

Many systems are known in the art for producing image amplification andconversion. One such system utilizes a sandwich comprising,sequentially, a transparent electrode layer, a photoconductive layer, anopaque electrically conducting layer, an electroluminescent layer and asecond transparent electrode layer, said electrodes being connected to asource of electrical potential. It should be noted here that it is acharacteristic of photoconductive materials that their electricalresistance decreases with exposure to electromagnetic radiation thuspermitting increased current iiow, the magnitude of said current beingdirectly proportional to the intensity of the incident radiation. Itshould also be noted that it is a characteristic of electroluminescentmaterials that their luminescence increases with increases in thedensity of electrical current flowing through them. It follows,therefore, that when a radiation image impinges upon the surface of thephotoconductive layer of the image amplifier being described, with avoltage applied across the transparent electrodes, current liows throughthe irradiated portions of the photoconductive layer an-d through theelectroluminescent layer to produce a replica of the original lightimage in the electroluminescent layer. That portion of the current whichis produced by the incident light is commonly referred to asphotocurrent.

Another system of image amplification known in the art utilizes the heatproduced by the ow of electrical current through a photoconductive layerand an excited thermosensitive phosphor layer to modulate the lightemission or fluorescence of the excited thermosensitive phosphor layer.The light emission of the phosphor layer may increase or decrease withincreases in temperature depending upon the composition of theparticular phosphor employed in the layer. This system of imageamplication employs a sandwich comprising, sequentially, a transparentelectrode layer, a photoconductive layer, a thermosensitive phosphorlayer and a second transparent electrode layer, said electrode layersybeing connected to a source of electrical potential and the phosphorlayer being excited by an externally applied source of stimulating3,479,515 Patented Nov. 18, 1969 ICC radiation, e.g., an ultravioletlamp. When an image of electromagnetic radiation impinges upon thesurface of the photoconductive layer of such a system current owsthrough the irradiated portions of the photoconductive layer and throughthe phosphor layer to produce a heat pattern in the phosphor layer of amagnitude and distribution `dependent upon the intensity anddistribution of the incident radiation and the product of the electricalresistance and the square of the current iiowing through thephotoconductor and phosphor layers. This heat pattern produces a lightpattern in the phosphor layer which may be either a positive or anegative replica of the original light image depending upon whether theuoresence of the phosphor layer is quenched or stimulated by the localincreases in temperature produced by the heat pattern.

An examination of the image amplifiers described above, one employing anelectroluminescent layer in combinatioh with a photocon-ductor layer,and the other ernploying a thermosensitive phosphor in combination witha photoconductor layer, reveals that an improved image amplifier mightbe obtained by combining the principles of both image amplifiers into asingle amplier, that is, by utilizing the current to stimulate theelectroluminescent layer and the heat generated by such current flow tomodulate the uorescence of the excited thermosensitive phosphor layer,thereby producing both an electroluminescent and a phosphorescentreplica of the original incident light image. If these principles arecombined and the individual layers arranged satisfactorily, theelectroluminescent image generate-d by the photocurrent will .stimulatethe photoconductive layer to generate additional photocurrent which willproduce more electroluminescence, etc., so that as a consequence of thisoptical feedback a small amount of incident radiation will produce arelatively large amount of photocurrent and therefore a relatively largeamount of heat to modulate the uorescence of the excited thermosensitivephosphor layer.

Such an improved image amplilier is contemplated by the presentinvention which is a preferred species of the invention of co-pendingapplication Ser. No. 442,696 of Nelson R. Nail. According to the latterinvention, current is passed through and heats an imagewise radiatedphotoconductor which in turn imagewise heats a thin heat conductiveelectrode which conducts the heat to a thermosensitive phosphor coatedon the outside of the thin electrode, The phosphor is preferably onewhich iiuoresces under actinic radiation and whose fluoroescence isthermally quenched, 'but it may be one which is preexcited by actinicradiation, non-fluorescent, and whose phosphorescence is stimulated byheat. According to the present invention, an electroluminescent layer isincluded between the photoconduetor and the heat conductive electrode.The imagewise current flow excites the electroluminescent layer, but theemitted light is not seen as in the electroluminescent system describedhereinabove. Instead, the light from the electroluminescent layer is fedback to further illuminate the photoconductor thus producing greatercurrent liow. Thus, it acts as an intensifying screen excited by thecurrent rather than by the original radiation. One might expect theelectroluminescent feedback to spread the light and reduce resolution inthe image. One might also expect the extra layer to contribute to thespread of the heat in the two layers between the electrodes, which inturn would be added to the spread of the heat in the electrode itselfall of which would reduce resolution of the image in the outsidethermosensitive phosphor layer. One surprising result is that the`deterioration of resolution is not great and the increase in contrastis well worth any such loss that in incurred.

When the thin electrode is made transparent and the thermosensitivelayer is a pre-excited thermosti'mulated one, the electroluminescenceadds to and reinforces the thermoluminescence. However, stimulatedphosphorescence is generally less contrasty than quenched fiuorescence,and transparent electrodes have higher electrical resistance which cutsdown on current flow and hence on the contrast of the heat image. Thusthe opaque electrode embodiments are much preferred. In the preferredarrangements, therefore, the electroluminescent image and the heat imageare positive relative to the original radiin contrast is well worth anysuch loss that is incurred. image is negative.

It is thus an object of this invention to provide an improved imageamplifier which is highly efficient and which produces substantial imageamplification.

It is a further object of this invention to provide an improved imageamplification system which gives negative or positive images withrespect to the applied image.

It is yet another object of this invention to provide an improved imageamplifier which utilizes electroluminescent image feedback to producegreater image amplification.

It is still another object of this invention to provide an improvedimage amplifier which can operate at very high output intensity levels.

These and other objects will be evident from the following descriptionand the accompanying drawing which is a schematic view of the layerarrangement of a .preferred embodiment of the image amplifier of thepresent invention.

Referring to the accompanying drawing layer 11 comprises a transparentelectrode, layer 12 comprises a photoconductive layer, layer 13comprises an electroluminescent layer, layer 14 comprises an opaquelayer having a high degree of heat and electrical conductivity and layer15 is an excited thermosensitive phosphor layer which may optionally becovered with a protective transparent layer. For optimum efficiency ofthe system it is imperative that the sandwich comprising the layers 11through 15 be made to have as low a thermal capacity as is consistentwith the optimum efficiency of the individual layers.

In the operation of this embodiment of the image amplifier of thepresent invention an alternating electrical potential is applied betweenelectrodes 11 and 14 and the phosphor layer 15 is excited by an externalsource of actinic radiation. When an image comprising a pattern ofelectromagnetic radiation is cast upon the photoconductive layer 12,locally decreasing its electrical resistance, current flows through theirradiated portion of the layer generating a heat pattern in the layerin accordance with the energy equation where P is the power dissipated,I is electrical current and R is the electrical resistance f layer 12.The heat pattern generated in the exposed areas of the photoconductivelayer is then transmitted through the thin opaque conductive layer 14,which has little lateral thermal conductivity due to the temperaturegradients of the system, to the excited thermosensitive phosphor layerto either quench its fluorescence or intensify it depending upon thecharacteristics of the phosphor employed. Thus, either a negative or apositive amplifier replica of the original image may be produced inaccordance with the type of fheremosensitive phosphor employed; aphosphor the fluorescence of which is quenched on heating affording anegative replica of the original image and a phosphor the uorescence ofwhich is stimulated by heat affording a positive replica of the originalimage.

In a preferred embodiment of the present invention, a layer ofphotoconductive copper-doped cadmium sulfide in an epoxy resin wascoated at a thickness of about 0.007 inch on an electrode comprisingstannic oxide strips of about 0.001 inch thickness, 0.010 inch width,and with a center-to-center distance of about 0.025 inch on glass.

The cadmium sulfide photoconductive layer was overcoated with an 0.001inch layer of an electroluminescent material such as manganese-dopedzinc sulfide in a silicone alkyd resin. A` silver paste was then appliedin a thin layer over the electroluminescent layer. A thermosensitivephosphor coating comprising about 49% of zinc sulfide, 49% of cadmiumsulfide, 2% of sodium chloride, 400 parts per ,million of silver and 2parts per million of nickel in a non-fluorescent binder was then appliedover the silver paste. The phosphor layer of the resulting sandwich wasthen irradiated with ultraviolet light t0 produce a uniform glow offluorescent light which was reduced slightly in intensity on theapplication of volts AC at a frequency of from 50 cycles to l0kilocycles across the silver paste and stannic oxide glass electrodes(due to the small amount of dark current flowing in the photoconductivelayer). An image of white light was then applied to the photoconductivecadmium sulfide layer through the stannic oxide glass electrode toproduce in the thermosensitive phosphor layer an image which wasnegative with respect to the applied image, the fluorescence of thephosphor layer being quenched in the areas corresponding to theilluminated areas of the photoconductive layer. Since the brightness ofthe fluorescent image in the thermosensitive phosphor layer wassignificantly greater than the brightness of the image applied to thephotoconductive layer, image amplification was accomplished.

While the preferred embodiment described above produced a negativereplica of the applied image in the thermosensitive phosphor layer, itis equally possible t0 produce a positive replica of the applied imageby using a thermosensitive phosphor in the image amplifier which storesexcitation energy and subsequently releases it in the form of visiblelight when the temperature of the phosphor is increased.

As was mentioned above, it is imperative in the construction of theimage amplifier that the sandwich comprising layers 11 through 15 asdesignated on the accompanying drawing be made to have as low a thermalcapacity as is consistent with the optimum efiiciency of the individuallayers if maximum sensitivity and efficiency are to be obtained. Forexample, the low thermal capacity transparent electrode layer couldcomprise a thin transparent conductive coating of evaporated metal on atransparent support having a low thermal capacity such as a thin film ofpolyethylene terephthalate. The photoconductive layer might be a thinlayer of any suitable photoconductive material such as cadmium sulfide,zinc oxide, amorphous selenium, lead sulfide, antimony sulfide, leadselenide, arsenic selenide, etc. By proper choice of the photoconductivematerial employed, the image amplifier may be made responsive toelectromagnetic radiations other than visible radiation, e.g., longX-rays, ultraviolet rays, near infrared rays, etc. It is equally obviousthat with certain wavelengths of radiation not visible to the eye whichproduce heat on absorption in various materials, the photoconductivelayer may be replaced by a layer which absorbs such radiation.

It is to be noted that as used in the specification and claims, the termamplify and its derivatives such as amplifying, amplification andamplifier are deemed to embrace not only amplification of a particulartype of electromagnetic radiation, but also the conversion of an imageof one type of electromagnetic radiation to an image of another type ofelectromagnetic radiation. For example, on the one hand, according tothe present invention, an incident image of visible radiation may heamplified to produce an image of visible radiation of greater intensityor, on the other hand, an incident infrared image, ultraviolet image,etc., may be converted to an image of visible radiation, etc.

The thermosensitive phosphors useful in the practice of this inventionmay be selected from proprietary products such as those sold by theUnited States Radium Company or they may be prepared by methods known inthe art. It is obvious that, by the .proper selection of thethermosensitive phosphors, the color of the amplified image, thebrightness of the image, and the heat input energy requirements may allbe varied to suit a particular application of the image amplifier.

Although specific embodiments of this invention have been described andillustrated above, it is obvious to those skilled in the art that otherembodiments are possible which are within the scope of the presentinvention as described above and in the appended claims.

What is claimed is:

1. A method of amplifying electromagnetic radiation energy comprisingthe steps of casing an image of such radiation energy upon means forconverting incident electromagnetic radiation energy into electricalconductivity, impressing an electrical potential across saidelectromagnetic radiation energy converting means in order to produce anelectrical current pattern within such radiation converting meanscorresponding to the image of electromagnetic radiation cast thereon,causing said electrical current pattern to pass through means forconverting electrical energy into electromagnetic energy such that saidelectromagnetic energy produced by said electrical current is cast backupon said electromagnetic energy converting means, said electricalcurrent pattern also producing a corresponding thermal image in saidelectromagnetic radiation converting means, and utilizing said thermalimage to produce a light image in means for converting a thermal energypattern into a light energy pattern, said thermal energy patternconverting means having no electrical current passing therethrough.

2. An electromagnetic radiation energy amplification device comprisingmeans for converting electromagnetic radiation energy into electricalconductivity, means for converting electrical energy intoelectromagnetic radiation energy, means for impressing an electricalpotential across said electromagnetic radiation energy converting meansand said electrical energy converting means to cause an electricalcurrent to pass through said respective means, the passage of currentthrough said electrical energy converting means causing the productionof thermal energy therein, and means, in thermal contact with saidelectrical energy converting means, for converting said thermal energyinto light energy, said thermal energy means having no electricalcurrent passing therethrough.

3. An electromagnetic radiation energy amplication device comprising atransparent electrode layer, a photoconductive layer, anelectroluminescent layer, an opaque heat conductive electrode layer, athermosensitive phosphor layer in thermal contact with said opaqueelectrode layer, a source of electrical potential connected across saidelectrode layers and an external source of actinic radiation to excitesaid thermosensitive phosphor layer, said phosphor layer beingpositioned relative to said opaque electrode layer so that currentflowing between said electrode layers does not flow through saidthermosensitive phosphor layer.

4. The electromagnetic radiation energy amplification device of claim 3wherein said thermosensitive phosphor layer comprises a phosphor thefiuorescence of which is quenched by heat.

5. The electromagnetic radiation energy amplification device of claim 3wherein said thermosensitive phosphor layer comprises a phosphor whichstores energy which is released as light energy on heating.

6. An electromagnetic radiation energy amplification device comprising asandwich including at least five layers positioned relative to eachother in the following positions:

a transparent electrode layer as a first layer,

a photoconductive layer as a second layer,

an electroluminescent layer as a third layer,

an opaque heat-conductive electrode layer as a fourth layer, and athermosensitive phosphor layer as a fifth layer, whereby, when anelectrical potential is impressed across said electrode layers and thethermosensitive phosphor layer is excited by an external source ofactinic radiation, an image of electromagnetic radiation cast upon saidphotoconductive layer through said transparent electrode layer producesan electrical current pattern, which passes between said electrodes butnot through said phosphor layer, the density of current at any pointbeing a function of the intensity of the radiation cast upon saidphotoconductive layer at that point,

said current pattern generates light in said electroluminescent layer,said light being cast back upon said photoconductive layer in accordancewith said image of electromagnetic radiation to intensify said currentpattern,

said current pattern also produces a thermal image corresponding to saidcurrent pattern and said radiation image, said thermal image beingtransferred to said thermosensitive phosphor layer to cause a lightenergy pattern to be emitted by said thermosensitive phosphor layer, theintensity of said emitted light being substantially greater than theintensity of the electromagnetic radiation image cast upon saidphotoconductive layer.

7. The electromagnetic radiation energy amplification device of claim 6wherein said thermosensitive phosphor layer comprises a phosphor thefluorescence of which is quenched by heat.

8. The electromagnetic radiation energy amplification device of claim 6wherein said thermosensitive phosphor layer comprises a phosphor whichstores energy which is released as light energy on heating.

9. The electromagnetic radiation energy amplification device of claim 6wherein said thermosensitive phosphor layer comprises a mixture of about49% of zinc sulfide, 49% of cadmium sulfide, 2% of sodium chloride, 400parts/million of silver, 2 parts/million of nickel and a non-fluorescentbinder.

UzS. Cl. X.R.

